The present invention is a technique for coherent beam combination of the output of multiple phase-locked optical amplifiers or lasers, using double-coated glass mirrors or mirror pairs in a non-planar configuration.
The peak far-field intensity produced by M phase-locked optical amplifiers or lasers of equal power can ideally be M times as great as the intensity produced by the same sources with random relative phases. This fact has motivated a great deal of work on achieving coherently phased optical sources. An example is a device that phases the output of 19 phase-locked diode lasers by sending this light through single-mode optical fibers. (L. Bartelt-Berger, U. Brauch, A. Giesen, H. Huegel, and H. Opower, Power-scalable system of phase-locked single-mode diode lasers, Appl. Opt. 38, 5752-5760 (1999)) Piezoelectric transducers are used to stretch the fibers so as to shift the optical phases. The optimal phases are maintained by means of an electronic feedback circuit.
Phase control is only one aspect of the problem of coherent beam combination. Generally the far field of a phased array of emitters has power distributed partially into side lobes, so that the central peak contains only a fraction of the total power. To some extent this problem can be reduced by the use of microlens arrays which collimate the light from the individual sources when it has diffractively spread almost to the point of overlapping. (J. R. Leger, M. L. Scott, and W. B. Veldkamp, Coherent addition of AlGaAs lasers using microlenses and diffractive coupling, Appl. Phys. Lett. 52, 1771-1773 (1988). This increases the filling factor of the array, which is the ratio of the sub-beam diameters at their waists to the distance between beams. A large filling factor results in a greater fraction of far-field power in the central lobe. However, a microlens array does not produce a Gaussian beam, and the beam quality is not diffraction limited.
In a pending patent application (Ser. No. 09/558,527 filed May 26, 2000, (now U.S. Pat. No. 6,310,715), allowed and hereby incorporated by reference) a technique was described using a stack of birefringent crystal plates to produce a coherently combined (multiplexed) polarized Gaussian beam which is diffraction limited. The present invention serves the same function and is conceptually similar, but requires only coated glass mirrors, which can be economically manufactured with large dimensions of extremely transparent well-tested materials such as fused silica or Infrasil. By contrast, large highly birefringent crystals may be unavailable, expensive, or subject to damage at high optical power. Other advantages of the present invention based on glass mirrors will be pointed out below.
The present invention is a multiplexing device for coherently combining the output of 2N narrow-bandwidth, diffraction-limited, polarized, and phase-locked optical beams to produce a single diffraction-limited beam with a power close to 2N times that of a single beam. The multiplexing device is comprised of N double-coated planar mirrors or coated mirror pairs, the top coating being designed to reflect the s-polarization and the bottom coating being designed to reflect the p-polarization or arbitrary polarization of narrow-bandwidth light incident at a selected wavelength and non-normal angle of incidence. The preferred angle of incidence using the coated mirror pairs is the Brewster angle. The mirror thickness or gap between paired mirrors is selected so that the walkoff between s and p waves increases by a factor of 2 in successive stages of reflection. The mirrors are arranged in a non-planar configuration. The 2N incident beams are arrayed transversely in a particular configuration and propagate all in the same direction. A multiplexer system is also disclosed that in combination with the multiplexing device controls the relative phases of the incident beams using an electronic feed-back circuit which monitors the power of certain undesired emitted beams and minimizes their power by phase adjusters such as piezoelectric fiber stretchers.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.